The global battle to reshape the $38 trillion bond market began before sunrise on Friday, Sept. 26, when an announcement blinked onto computer screens in the Newport Beach, Calif., offices of Pacific Investment Management Co.

Bill Gross, the firm’s co-founder and chief investment officer, arrived that morning around 5 a.m., earlier than usual, but he didn’t stay long or speak to anyone, according to people familiar with what happened. Just before 5:30 a.m., traders stared at their screens in disbelief.